This patent application claims priority from a Japanese patent application No. 2000-327262 filed on Oct. 26, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a glass base material grinding apparatus and a method for manufacturing a glass base material. More particularly, the present invention relates to a grinding apparatus and a method for manufacturing a glass base material having an excellent degree of circularity and also having an excellent eccentricity of a core.
2. Description of the Related Art
A porous glass base material, which is a base material of an optical fiber, is usually manufactured by accumulating glass particles on a surface of a core member by using a method such as the VAD (Vapor-phase Axial Deposition) method, or the OVD (Outside Vapor Deposition) method. A glass base material is manufactured by dehydrating and sintering the porous glass base material. The core member becomes a core of a glass base material after the glass base material is dehydrated and sintered. A preform is formed by elongating a glass base material, and an optical fiber is manufactured by drawing a preform.
As a method for increasing an accumulation speed of the glass particles on the surface of the core member in the OVD method, there are a method of using a burner having a large bore diameter and a method of increasing the number of burners. The burner ejects glass particles and accumulates glass particles on a surface of a core member. Furthermore, as a method for increasing the productivity of porous glass base material in the OVD method, there is a method of increasing the length of the core member to increase the ratio of the straight body part in the glass base material product. The straight body part has a uniform diameter.
The method of increasing the accumulation speed of the glass particles by increasing the bore diameter of the burner has a problem that the accumulation speed does not increase because the attachment ratio of the glass particles to the core member is extremely low at the initial process of the accumulation. Furthermore, if a plurality of burners are used, accumulation efficiency does not increase because each flame of the burners interferes with each other.
On the other hand, the method for increasing the number of burners has a problem of causing unevenness of the surface of the accumulated body of glass particles. In particular, if increasing the amount of raw material gas supplied to the burner increases the accumulation speed, the unevenness of the surface of the accumulated body becomes very significant. As a result, the optical fiber drawn from the glass base material manufactured by the OVD apparatus using an increased number of burners does not have a good optical characteristic. For example, a single mode optical fiber cannot have a desired cutoff wavelength and a dispersion characteristic.
Furthermore, in a case of the method that increases the length of the core member, the core member may bend during accumulation of the glass particles because the length of the core member is long. Thus, the resulting product cannot be used as a glass base material.
As a method for decreasing the unevenness that occurs on the surface of the glass base material and matching the center position of the core with the center position of the glass base material, there is a method of grinding the glass base material. The method of grinding the glass base material to match the center position of the core member and the center position of the glass base material is disclosed in Japanese Patent Application Laying-Open No. H9-328328 and Japanese Patent Application Laying-Open No. 2000-47039.
However, the method disclosed in Japanese Patent Application Laying-Open No. H9-328328 and Japanese Patent Application Laying-Open No. 2000-47039 could not match the center position of the core member and the center position of the glass base material when the core member is bent throughout the longitudinal direction of the glass base material.
Furthermore, the methods disclosed in Japanese Patent Application Laying-Open No. H9-328328 and Japanese Patent Application Laying-Open No. 2000-47039 have a problem that a cutoff wavelength of the optical fiber, which is drawn from the glass base material, becomes uneven through the longitudinal direction of the glass base material according to the fluctuation of the diameter of a core member through the longitudinal direction of the glass base material. This problem occurs because the methods disclosed in Japanese Patent Application Laying-Open No. H9-328328 and Japanese Patent Application Laying-Open No. 2000-47039 grind the glass base material such that the diameter of the glass base material becomes constant throughout the longitudinal direction of the glass base material.
Furthermore, when the center position of the core member is different from the center position of the glass base material, the optical fiber obtained by drawing this glass base material causes a connection loss when each end of the two optical fibers are fused and connected to construct an optical fiber network.
Therefore, it is an object of the present invention to provide a method for manufacturing a glass base material and a glass base material grinding apparatus, which is capable of overcoming the above drawbacks accompanying the conventional art. The above and other objects can be achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.
According to the first aspect of the present invention, an apparatus for grinding a glass base material having a core and a clad comprises: a grinding wheel for grinding the clad; a measuring unit for measuring eccentricity between a center position of the glass base material and a center position of the core in a direction perpendicular to a longitudinal direction of the glass base material at a plurality of positions along a longitudinal direction of the glass base material; a design unit for calculating target diameters of the glass base material substantially continuous throughout the longitudinal direction of the glass base material by calculating the target diameters, a center position of said target diameter is the same as the center position of the core for each of said plurality of positions along a longitudinal direction of the glass base material, so that the eccentricity becomes substantially zero for each of the plurality of positions where the eccentricity is measured by the measuring unit; and a control unit for controlling the grinding wheel to grind the clad so that a diameter of the glass base material to be the target diameter, the center position of which is at the center position of the core, substantially continuous throughout the longitudinal direction of the glass base material based on the target diameters calculated by the design unit.
The design unit may calculate the target diameter substantially continuous throughout the longitudinal direction of the glass base material by calculating the target diameter at a position between the plurality of positions where the eccentricity is measured by the measuring unit based on the eccentricity measured at the plurality of positions by the measuring unit. The design unit may calculate the target diameter at a position between the plurality of positions using the least-squares method.
The control unit may grind the clad by moving the grinding wheel back and forth in the direction toward the center of the glass base material. The control unit may rotate the glass base material around the axis of the glass base material and may move the grinding wheel back and forth toward the center of the glass base material so that movement of the grinding wheel against the glass base material forms a sine curve with an increase in an amount of rotation of the glass base material.
A number of the plurality of positions for measuring the eccentricity along a longitudinal direction of the glass base material may be substantially more than twenty.
The design unit may calculate the target diameters at each of the plurality of positions and the positions between the plurality of positions so that a ratio between a diameter of the core and a diameter of the glass base material becomes substantially constant throughout a longitudinal direction of the glass base material.
The grinding wheel may include: a coarse grinding wheel having a coarse surface; a fine grinding wheel having a fine surface; and the control unit grinds the clad using the fine grinding wheel after grinding the clad using the coarse grinding wheel.
The apparatus may further comprise a plurality of the grinding wheels, wherein the grinding wheels are arranged parallel along a longitudinal direction of the glass base material.
According to the second aspect of the present invention, a method for manufacturing a glass base material having a core and a clad comprises: accumulating glass particles around a core member, which becomes the core, to form a porous glass base material; dehydrating and sintering the porous glass base material to form a glass base material; measuring an eccentricity between a center position of the glass base material and a center position of the core in a direction perpendicular to a longitudinal direction of the glass base material at a plurality of positions along a longitudinal direction of the glass base material; calculating target diameters of the glass base material substantially continuous throughout the longitudinal direction of the glass base material by calculating the target diameters, a center position of said target diameter is the same as the center position of the core for each of said plurality of positions along a longitudinal direction of the glass base material, so that the eccentricity becomes substantially zero for each of the plurality of positions where the eccentricity is measured by the measuring; and grinding the clad with a grinding wheel so that a diameter of the glass base material to be the target diameter, the center position of which is at the center position of the core, substantially continuous throughout the longitudinal direction of the glass base material based on the target diameters calculated substantially continuous throughout the longitudinal direction of the glass base material.
Calculating may calculate the target diameter substantially continuous throughout the longitudinal direction of the glass base material by calculating the target diameter at positions between the plurality of positions where the eccentricity is measured by the measuring based on the eccentricity measured at the plurality of positions. The calculating step may calculate the target diameter at positions between the plurality of positions using the least-squares method.
The grinding may grind the clad by moving the grinding wheel back and forth in the direction toward the center of the glass base material. The grinding may rotate the glass base material around the axis of the glass base material and may move the grinding wheel back and forth toward the center of the glass base material so that movement of the grinding wheel against the glass base material forms a sine curve with an increase of an amount of rotation of the glass base material.
The measuring may measure the eccentricity along a longitudinal direction of the glass base material for more than twenty places along a longitudinal direction of the glass base material. The calculating may calculate the target diameters at each of the plurality of positions and the positions between the plurality of positions so that a ratio between a diameter of the core and a diameter of the glass base material becomes substantially constant throughout a longitudinal direction of the glass base material.
The grinding may grind the clad by a fine grinding wheel, which has a fine surface, after grinding the clad by a coarse grinding wheel, which has a coarse surface. The grinding may grind the clad using a plurality of the grinding wheels arranged parallel along a longitudinal direction of the glass base material.
The summary of the invention does not necessarily describe all necessary features of the present invention. The present invention may also be a sub-combination of the features described above. The above and other features and advantages of the present invention will become more apparent from the following description of the embodiments taken in conjunction with the accompanying drawings.